The present invention relates to a method of extruding thin-walled tubular bodies of ceramic powders. Such bodies are espeically useful for forming coaxial capacitors.
Coaxial capacitors comprise a tube of ceramic material such as BaTiO.sub.3, SrTiO.sub.3 or the like. Inner and outer electrodes are formed on the tube so as to be electrically insulated from each other. Means such as metal cups can be affixed to the ends of the tubular body to make electrical connection to the respective electrodes. Such capacitors and methods of making the same are taught in U.S. Pat. Nos. 4,109,292 and 4,197,570.
The volumetric efficiency of such capacitors can be increased by decreasing the wall thickness between electrodes and by increasing the electrode surface area by means such as employing a more intricate cross-sectional configuration. In the extrusion process by which the ceramic tubes are made, these geometric constraints which are needed to increase capacitance are disadvantageous in that they result in a very fragile wet extrudate. The extrudate gradually sags to a useless shape unless it can become sufficiently hardened to be self-supporting.
In the art of manufacturing bodies for coring and moulding in founding, rapid curing has been accomplished by combining the sand, particulate oxide or the like with a synthetic resin containing an acid curing agent such as a resin of the urea/formaldehyde, phenol/formaldehyde, furane or furance alcohol or by unsaturaated or epoxidised compounds. U.S. Pat. No. 3,145,438 teaches that the injection of SO.sub.3 gas into the mixture of sand and resin causes an instantaneous curing of the resin in the region subjected to the action of the SO.sub.3. However, such instantaneous curing impedes the diffusion of the SO.sub.3 to the other parts of the mixture. The injections of SO.sub.2 into the sand-resin mixture results in a very slow reaction at ambient temperature. Only a small amount of sulfur dioxide is converted to sulfuric acid by moisture in the air.
It is proposed in U.S. Pat. No. 3,879,339 that the granular filler be mixed with at least one acid curing organic resin and an oxidizing agent. After shaping the resultant mixture, the resin is cured at ambient temperature by adding gaseous or aerosol sulfur dioxide. The oxidizing agent reacts with the sulfur dioxide to form sulfuric acid in situ without forming sulfur trioxide, the action of which is said to be too violent and lacking selectively. The main aim of the invention disclosed in U.S. Pat. No. 3,879,339 is thus to form within the composition a compound which has a level of oxidation with lower reactivity than that of sulfur trioxide, which has the double disadvantage of being unstable and of combining with water to give an excessively violent reaction. Hydration of the sulfur dioxide by trace quantities of water takes place initially within the composition, followed by oxidation of the resultant sulphurous acid (or possibly hydrated SO.sub.2), either by formation of free radicals or by formation of complexes. In every case, sulfuric acid forms in situ without the intermediate formation of sulfur trioxide.